1. Field of the Invention
The present invention relates to a GPS (global positioning system) position-measuring device, a GPS position-measuring method, and a GPS position-measuring program which measure the position of a moving object by utilizing a GPS and also to a navigation system using the device, method, and the program.
2. Description of the Related Art
Position measurement utilizing GPS satellites (the measurement will hereinafter be referred to as “GPS position measurement”) is widely used as a method for determining the positions of moving objects, such as motor vehicles. In the GPS position measurement, when three GPS satellites can be tracked, the two-dimensional position (the latitude and longitude) of the moving object can be measured, and when four or more GPS satellites can be tracked, the three-dimensional position (the latitude, longitude, and altitude) of the moving object can be measured. In the GPS position measurement, the number of GPS satellites that can be tracked varies depending on the position-measuring environment. Thus, a method that uses the altitude obtained by the most-recent three-dimensional position measurement is also available when only the two-dimensional position can be measured.
Japanese Unexamined Patent Application Publication No. 5-45436 discloses a GPS receiver having an atmospheric-pressure sensor. When only two-dimensional position measurement can be performed, the GPS receiver uses altitude information determined from an atmospheric pressure to perform three-dimensional position measurement.
Japanese Unexamined Patent Application Publication No. 2000-275051 discloses a navigation apparatus. This navigation apparatus determines altitude information from barometric-pressure data. When multiple roads having different altitudes from each other exist on a road map, the navigation apparatus selects a road that is close to the determined altitude information to perform matching of the vehicle on the map.
Japanese Unexamined Patent Application Publication No. 2006-214993 discloses a moving-object navigation apparatus. This moving-object navigation apparatus determines the position/speed of a moving object by measuring a barometric pressure, detecting the altitude of the moving object from the measured barometric pressure, and correcting the detected latitude based on the moving-object altitude obtained through GPS position measurement. When the GPS position measurement cannot be performed, the moving object navigation apparatus uses the altitude detected from the barometric pressure.
Japanese Unexamined Patent Application Publication No. 9-61269 discloses a barometer for measuring a barometric pressure including only changes originating from a weather condition by removing a barometric-pressure change originating from an altitude change.
As described above, GPS position measurement is used to determine the position of a moving object. However, a GPS receiver generally uses all signals of GPS satellites that can be tracked to perform position-measurement computation. For example, when the GPS receiver can track five or more GPS satellites, it determines GPS-satellite combinations therefrom and uses the average value of the three-dimensional positions of the determined combinations as the measurement position or selects a specific GPS-satellite combination based on the reception strengths and/or the elevation angles of GPS satellites and uses the three-dimensional position of the selected combination as the measurement position.
The GPS position measurement, however, has a drawback in that it is easily affected by the position-measurement environment. For example, when the moving object travels in an urban area where there are high-rise buildings and so on, the GPS receiver may receive multipath signals arriving from GPS satellites. The GPS receivers of the related art cannot determine signals from which GPS satellite are affected by multipath, and thus performs position measurement by using signals from a GPS satellite affected by the multipath. Consequently, there are problems in that the position measurement of the moving object has a large error and the accuracy of the GPS position measurement deteriorates.